1. Field of the Invention
This invention relates generally to automatic test equipment (ATE) and accompanying processes for testing electronic devices, and, more particularly, to test equipment and processes for testing characteristics of high-speed bit streams.
2. Description of Related Art
Manufacturers of integrated circuits commonly use automatic test equipment (ATE) to verify devices at different manufacturing stages. Through the use of ATE, manufacturers can identify device faults early in the manufacturing process, thus avoiding the costs of performing additional manufacturing steps on defective devices. Manufacturers may also use ATE to grade devices across different levels of performance. As better-performing devices generally fetch higher prices, the ability accurately to test integrated circuits translates directly into increased profits.
A primary goal of ATE is to test electronic devices quickly and accurately. As devices become faster and more complex, ATE must advance to keep pace with these improvements.
The popularity of Serializer/Deserializer, or “SerDes,” and other high-speed serial devices has greatly increased in recent years with improvements in telecommunications and networking technologies. SerDes devices convert parallel bit streams into serial bit streams that change at a multiple of the input, parallel data rate. They may also perform the reverse function of deserializing serial bit streams, by converting them into parallel bit streams that change at a fraction of the serial data rate. SerDes devices are now available at serial data rates up to 13 GB/s (billion bits per second), and 40 GB/s parts will soon be available. By comparison, state-of-the-art testers can commonly generate digital waveforms at speeds of only several hundred megahertz. This still falls short of the speed needed to directly test the fastest currently available SerDes devices at full speed.
Previous attempts at measuring high-speed serial bit streams have included using specialized test instruments such as TJD's (Time Jitter Digitizers). TJD's detect edges of electrical signals at their inputs and apply time-stamp values indicative of the times at which the edges occur. TJD's tend to be costly, multifunctional instruments, however, offering features that extend well beyond those needed for serial testing. They also tend to operate at slower speeds than those required for testing the fastest SerDes devices.
Another prior technique for testing serial bit streams is disclosed in U.S. patent application Ser. No. 09/635,334, entitled “CAPTURING AND EVALUATING HIGH SPEED DATA STREAMS,” which is hereby incorporated by reference in its entirety. As described in that patent, a tester employs its digital driver circuits to apply parallel input data to a SerDes device, and employs a latching comparator to undersample the output of the SerDes device. The tester's detector circuits then sample the comparator's latched output to reconstruct the high-speed serial waveform. This technique is highly effective for measuring timing characteristics of serial bit streams. The technique tends to be exceedingly complex, however, both in terms of the programming instructions required to conduct the process and the computations needed to generate the measurement results.
What is desired is a simpler technique for measuring the timing characteristics of high-speed bit streams using automatic test systems.